image: add xlarge (4096px) on-demand preview tier

New `PhotoSize::XLarge` variant sits between `Large` (2048px) and
`Full` (original). On-demand generated and disk-cached at
`_xlarge/<hash>.jpg`, same waterfall as `Large` (embedded RAW preview
→ ffmpeg → image crate). Sources below 4096px serve at native size.

Reduces decoded bitmap memory from ~192MB (48MP full) to ~64MB for
the mobile viewer's zoom tier.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/content_hash.rs

@@ -62,6 +62,15 @@ pub fn large_preview_path(thumbs_dir: &Path, hash: &str) -> PathBuf {
         .join(format!("{}.jpg", hash))
 }
 
+/// Hash-keyed xlarge-preview path: `<thumbs_dir>/_xlarge/<hash[..2]>/<hash>.jpg`.
+pub fn xlarge_preview_path(thumbs_dir: &Path, hash: &str) -> PathBuf {
+    let shard = shard_prefix(hash);
+    thumbs_dir
+        .join("_xlarge")
+        .join(shard)
+        .join(format!("{}.jpg", hash))
+}
+
 /// Hash-keyed HLS output directory: `<video_dir>/<hash[..2]>/<hash>/`.
 /// The playlist lives at `playlist.m3u8` inside this directory and its
 /// segments are co-located so HLS relative references Just Work. See